In recent years, technical advances for ink jet recording have markedly progressed, and the image quality of ink jet prints has approached that of photographic prints, employing a printer technique, an ink technique and an exclusive recording medium. As the image quality has been improved, image storage stability has been also compared with that of photographic prints. Many dye inks used in ink jet recording are pointed out to have problems in that the formed image lowers its quality because dye movement occurs due to poor water resistance or poor resistance to bleed, or chemical reaction specific to the dye due to poor light fastness or poor resistance to oxidizing gases occurs.
There have been many proposals to improve storage stability of dye ink images. For example, in Japanese Patent Publication No. 2-31673, water resistance, weather resistance, and image glossiness has been improved by recording an image on a recording medium comprising an uppermost layer containing thermoplastic organic polymer particles, and then heating the resulting medium to melt the thermoplastic particles and form a film, whereby a polymer protective layer is formed on the surface.
This technique has been continuously studied and put into practical use, but is still poor in various aspects as compared with the silver halide photographic technique.
Further, as a result of technical progress in recent years, the image quality of ink jet prints prepared by utilizing a dye ink has approached those prepared utilizing silver halide photography, and further, the price of such units has been reduced. As a result, ink jet printing has been increasingly employed.
A first problem is ink absorption. In recent years, image quality of ink jet prints has more and more approached that of silver halide photographic prints by marked technical progress of ink jet printers, the use of smaller-sized ink droplets, a multi-dot technique, light colored inks with a low dye concentration, and the like. Further, printing speed has been increased, the number of ink nozzles has been increased, scanning speed has been increased, and recording heads arranged in a line has predominated. Unless a recording medium has sufficient ink absorption speed or ink absorption capacity under such conditions, color bleed or beading occurs, resulting in lowering of image quality. Color bleed herein referred to is what is generally called color blur. Beading herein referred to is one visually recognized as color unevenness in the form of small circles, which is likely to occur due to the low speed of ink absorption in an ink absorption layer. This phenomenon probably occurs due to poor ink absorption (ink penetration) of an upper ink absorption layer comprised of thermoplastic organic polymer particles.
In contrast, a method is disclosed in Japanese Patent O.P.I. Publication No. 2000-203151 which increases the particle size of thermoplastic organic polymer particles, however, the size increase requires more time to melt the particles by heating to form a film, which is an obstacle to shortening the total processing time, which includes an ink jet recording time shortened due to an increase of recording speed increased by improving the ink jet technique. Further, there is a proposal in Japanese Patent O.P.I. Publication No. 7-237348 which adds silica particles to an uppermost layer comprising thermoplastic organic polymer particles in order to minimize beading. However, in the proposed technique, some of the silica particles are buried in the voids formed among the thermoplastic organic polymer particles, resulting in lowering of ink absorption speed. An attempt to adjust the particle distribution in order to improve the ink absorption has been insufficient, since the silica particles are contained in only a limited amount as an additive. A method is disclosed in Japanese Patent O.P.I. Publication No. 2000-280603 in which (coloration) is improved by adding colloidal silica particles in an amount of at most 30% to an uppermost layer comprising thermoplastic organic polymer particles, but this has not provide the effect of improving the ink absorption property.
A second problem is image storage stability. An ink jet recording medium employing the technique of melting thermoplastic organic polymer particles to form a film provides improved water resistance or bleed out resistance, however, the light fastness is not sufficient as compared with that of silver halide photosensitive materials. A proposal is made which records an image on an ink jet recording medium comprising an uppermost layer containing thermoplastic organic polymer particles employing pigment ink capable of improving image storage stability, particularly, light fastness, but does not simultaneously solve another problem. In the disclosure in Japanese Patent O.P.I. Publication No. 11-157207, ink absorption speed is low, and a drying time of 30 minutes is required after ink jet recording, resulting in overall lowering of total throughput. An uppermost ink absorption layer containing thermoplastic organic polymer particles in an ink jet recording medium is disclosed in Japanese Patent O.P.I. Publication Nos. 11-192775 and 11-208097, but the disclosure does not provide sufficient ink absorption. A method is proposed in Japanese Patent O.P.I. Publication No. 2000-158803 in which the particle size of the thermoplastic organic polymer particles is adjusted to be not less than 1 μm, so that pigment particles in pigment ink do not clog ink paths (voids) of the ink absorption layer and also so that ink absorption is not lowered. The size increase requires much time to melt the thermoplastic organic polymer particles by heating to form a film, resulting in an obstacle to shortening of the total throughput (transporting time). A method is proposed in Japanese Patent O.P.I. Publication No. 2000-203152 in which inorganic pigment particles on the order of μm are added to the layer beneath the layer comprised of thermoplastic organic polymer particles. However, the use of inorganic pigment particles of that size lower image transparency, and does not provide the same quality as silver halide photosensitive materials.
A third problem is the total processing time. The total processing time herein referred to means the time required to record an image on a recording medium, and to melt an upper layer to form a film, including drying time and time necessary to melt the layer to form a film. A method is disclosed in Japanese Patent O.P.I. Publication No. 9-104164 which adds glass fibers with a high aspect ratio to an uppermost layer comprising thermoplastic organic polymer particles, in order to increase heat conductivity and reduce the energy required for film formation. However, the simple addition of the glass fiber cannot provide satisfactory ink absorption speed and therefore, is not suitable for high speed printing, and does not contribute to the desired total throughput time.
The above techniques have other problems as described below.
One problem is poor writability on an image. A recording medium comprising an uppermost layer comprised mainly of thermoplastic organic polymer particles is rather resistant in writability with a ball point pen or an aqueous felt-tip pen. That is, the recording medium has poor ink receptivity and is liable to produce stains when rubbing the surface.
Another problem is poor strength of the image surface. In a recording medium comprising an uppermost layer comprised mainly of thermoplastic organic polymer particles, the formed image is likely to be damaged due to the poor surface strength, and the damage is significant.
Still another problem is poor layer strength of the uppermost layer during a period from the time when forming an image to the time when melting the thermoplastic organic polymer particles of the layer to form a film. The thermoplastic organic polymer particles in the layer melt easily to form a film at a later step, and are not strongly combined with another. Most of such layers comprise aqueous latex as thermoplastic organic polymer particles, and have a structure of comparatively poor water resistance before film formation. Accordingly, a recording medium comprising an uppermost layer comprised mainly of thermoplastic organic polymer particles may produce flaws in the transport step, which results from the transport system in a printer after ink jet recording, or may result in damage by rollers on the surface. This phenomenon markedly occurs when it is used in a high speed printer.
A fourth problem is adhesion of an image. When a recording medium with an image is stored at high temperature (for example, in a warm room or a car in summer), covered with the protective sheet in an album, the polymer film on the image surface may adhere to the protective sheet. As the polymer used for thermoplastic organic polymer particles used in the uppermost layer used is a polymer with a high enough Tg that does not adhere to the sheet at ordinary temperature. However, the apparent Tg of such a polymer may be lowered by the action of a solvent in ink used for ink jet recording or a plasticizer in the protective sheet, and the recorded image may adhere to the protective sheet at high temperature that may be realized under normal environmental conditions.
A structure is disclosed in Japanese Patent O.P.I. Publication No. 7-101142 in which the gloss layer of a recording medium contains latex in a small amount and inorganic pigment, however, in this structure, the shape of the latex particles may vary during calender treatment in the manufacture process of the medium. There is, further, disclosed another structure which employs a complex emulsion containing an acryl resin and colloidal silica. This structure is different from the structure in which inorganic pigment and thermoplastic particles are mixed to form ink absorptive voids, but does not provide sufficient ink absorption resulting from inorganic pigment, nor sufficient gloss resulting from polymer components, nor sufficient image storage stability.
As a method of manufacturing a recording medium comprising an upper layer containing thermoplastic particles, a method is known in which an ink absorption layer with high ink absorption is coated on a substrate, and then a layer containing thermoplastic particles is coated on the resulting ink absorption layer. Although aqueous latex is often used as the thermoplastic particles in view of environmental concern, the aqueous latex ordinarily has a low viscosity, and therefore, has the problem that it is difficult to be coated on a substrate and dried. The viscosity can be increased by the addition of binders to the latex in order to overcome the above problem, but that results in lowering of ink absorption speed.
A recording medium with high gloss and high ink absorption is known which comprises silica particles and a hydrophilic binder. The recording medium is manufactured by simultaneously multi-layer coating method employing the viscosity difference among coating solutions, resulting in cost reduction, and has high gloss and high ink absorption, which is favorably accepted on the market. When a thermoplastic particle-containing layer is coated as an upper layer of the void type silica particle-containing recording medium, a silica particle-containing layer is coated on a substrate, and then a layer containing thermoplastic particles is coated on the resulting silica particle-containing layer, as described above. In this coating method, two coating processes and two drying processes are required, which has disadvantage in cost increase.